a) Field of the Invention
The present invention relates to a liquid crystal display device and its manufacture method, and more particularly to a liquid crystal display device having pre-tilt orientation slightly tilted in one direction from homeotropic orientation (configuration of liquid crystal molecules orientated vertical to the substrate surface) and its manufacture method.
b) Description of the Related Art
Homeotropic orientation is one of the operation modes of a liquid crystal display device. In this mode, liquid crystal molecules having negative dielectric anisotropy are orientated vertical to the substrate. As voltage is applied between the substrates, liquid crystal molecules tilt toward the substrate surface.
A radial direction about the substrate normal of liquid crystal molecules in homeotropic orientation is isotropic, and hence molecules tilt omnidirectionally about the substrate normal. To solve this, pre-tilt orientation has been proposed in which liquid crystal molecules are tilted slightly, in a state of no voltage application, in one radial direction about the substrate normal. Liquid crystal molecules with a pre-tilt angle in one radial direction about the substrate normal can be tilted in this one radial direction when voltage is applied.
The following methods are used for giving liquid crystal molecules a pre-tilt angle from the substrate normal toward the substrate surface.
(1) A homeotropic alignment film is formed on the surface of a substrate. The homeotropic alignment film is made of, for example, homeotropic orientation type polyimide (e.g., SE-1211 manufactured by Nissan Chemical Industries, Ltd., Japan) or silane coupling type homeotropic orientation material (e.g., DMOAP manufactured by Chisso Co., Japan). The surface of the homeotropic alignment film is thereafter rubbed with rayon cloth or the like.
As shown in FIG. 4A, CH chains designated by CH initially formed vertically on the surface S of a homeotropic alignment film P are slightly tilted by rubbing. The director d of each liquid crystal molecule ML tilts along the direction of the tilted CH chain. An angle between the substrate normal n and the director d of the liquid crystal molecule is called a pre-tilt angle .theta..sub.P.
(2) As shown in FIG. 4B, an oblique vapor deposition film or the like made of SiO or other materials is formed on the surface of a substrate to form an underlying layer U having an oblique structure. A homeotropic alignment film P such as described above is formed on the underlying layer U. The surface of the homeotropic alignment film P has an oblique structure conformal to the oblique structure of the underlying layer U. Although each CH chain on the homeotropic alignment film P is vertical to the surface S of the homeotropic alignment film P, it is inclined relative to the substrate surface. In this case, rubbing is not necessary.
With the above method (1), different pre-tilt angles are likely to be formed if rubbing is nonuniform. Fine stripe disorders are also likely to be formed by rubbing. These different pre-tilt angles and fine stripe disorders are visually recognized as abnormal patterns on the liquid crystal display when it is turned on (in a voltage application state). This may considerably degrade the image quality of the liquid crystal display.
With the above method (2), relatively uniform pre-tilt orientation can be obtained. However, the manufacture processes are complicated. Oblique vapor deposition of SiO or the like requires a vacuum vapor deposition system so that cost is raised and throughput is lowered. If a large substrate is used and the relative positions of the substrate and the oblique vapor deposition source material change during the deposition process, many different pre-tilt angles are formed. In order to avoid this, a robust and complicated deposition system is required.